I sprained my ankle 6 weeks ago. X-rays taken, no fractures. The outside is swollen by bone, how can I tell if its fluid or swelling for sure?

Swelling is fluid. The swelling results form the accumulation of fluid in the area. An MRI can show soft tissue swelling and soft tissue injuries.
MRI. If you want to know it is free fluids or swelling of the tissue, MRI is the best test for that.
Ultrasound. Musculoskeletal ultrasound is being used more often for detecting soft tissue abnormalities such as tendon tears, joint effusions, and cellulitis. Ultrasound would show fluid in tendon sheaths and joint vs subcutaneous soft tissue swelling. Radiographs show better abnormalities affecting bones such as fracture, tumor, and bone infection.